Field of the Invention
The present invention relates to an image processing apparatus that processes three-dimensional images obtained by various kinds of imaging apparatuses (modalities) such as a nuclear magnetic resonance imaging (MRI) apparatus, an X-ray computed tomography (X-ray CT) imaging apparatus, and an ultrasonic (US) image diagnostic apparatus, an image processing method, and a non-transitory computer-readable storage medium.
Description of the Related Art
In image diagnosis using three-dimensional images (three-dimensional tomographic images), a doctor makes a diagnosis while comparing images obtained using a plurality of imaging apparatuses (modalities) and different positions, times, and imaging parameters. However, since the object shape changes between the images, it is difficult to identify or compare the lesion. There is an attempt to do deformation alignment (deformation estimation) between a plurality of images. This makes it possible to deform one image and generate a deformed image in which the position or shape of an object visualized in the image almost matches that in another image. It is also possible to calculate and present the position of a point on the other image, which corresponds to a point of interest on the one image. As a result, the doctor can easily identify or compare the lesion between the plurality of images. In a field other than the medical field, the same operation as described above may be executed for the purpose of inspecting the internal state of an object using three-dimensional images.
At this time, an image includes various organs and body tissues, and tissue hardness changes depending on the type. For example, a bone is very hard, and therefore, its shape hardly changes even if the posture or shape of the object changes between images to be compared. There also exists a lesion such as a tumor that is harder than a tissue on the periphery depending on the type, and hardly changes the shape. Hence, if deformation alignment (deformation estimation) is performed by handling the hard tissue like the soft tissue on the periphery, the hard region that is not actually deformed may be estimated as deformed, and a wrong alignment result may be obtained.
As a solution to this problem, Japanese Patent Laid-Open No. 2013-141603 proposes a technique of, in a displacement field obtained by non-rigid body alignment, bringing only the displacement field of a hard region of interest that should be a rigid body close to rigid body transformation, thereby avoiding the region of interest from being erroneously estimated as deformed. More specifically, this literature describes a technique of approximating only the deformation of a region of interest to rigid body transformation, generating the displacement field of the approximated rigid body transformation for the region of interest, generating the displacement field of original deformation alignment for a region of non-interest, and generating a displacement field by spatially combining those displacement fields. A displacement field here means a field that holds transformation (displacement) between positions in two images. As described above, this literature discloses a technique of performing deformation alignment (deformation estimation) between images using a transformation model of a different characteristic depending on a region in an image. A transformation model here represents a model used to transform the coordinates of each position on an image in the alignment.
However, in the technique disclosed in Japanese Patent Laid-Open No. 2013-141603, coordinate transformation is represented using transformation models of different characteristics for a region of interest and a region of non-interest. Interpolation is performed, and the regions are combined, thereby attaining overall deformation alignment. Hence, a consistent deformation cannot be obtained particularly at the joint.